Flexible membrane linings have been used for many years to provide impermeable ground protection and retain liquids in excavated pits and reservoirs, and to transport water in irrigation ditches.
More recently, such lining systems have been used in the containment of hazardous waste materials and municipal landfills to prevent the leaching of chemicals or waste materials into the ground surfaces, whence they ultimately contaminate public or private water supplies.
Where the linings required are small enough in area to be handled and installed in one piece, they can be prefabricated of sections seam welded at the factory and transported to the field site. A prior welding machine suitable for factory welding is disclosed in U.S. Pat. No. 3,402,089. However, such machine requires a solid, smooth foundation such as a factory floor and is not suitable for seam welding large panels of lining over irregular terrain in the field.
A great many of the outdoor installations are quite extensive, covering areas of several acres, so that it is literally impossible to fabricate a flexible membrane liner as a one-piece liner at the factory and then transport and install it in the field. Most such applications require panels of not over 70 feet wide by 200 feet long and weighing less than 4000 pounds to be prefabricated in a factory environment, and the individual panels are then unrolled and installed in the field by overlapping and seaming their edges together.
In the past, sustantially all such field seaming has been accomplished by the use of solvents or adhesives to join the overlapping marginal portions together, as hot air or thermal seaming has been impractical, if not impossible, due to adverse field conditions such as wet, muddy, sandy or rocky soil conditions precluding the required smooth, solid backup pressure required for prior thermal seam-welding machines.
However, while solvent or adhesive systems have been, to some extent, satisfactory for the containment of water, they are not satisfactory or desirable for the containment of chemicals and hazardous waste materials. Solvent and adhesive seams are susceptible to defects in applicator techniques, and to environmental conditions at the time of application, such as moisture or cold temperatures. Whereas, a slight seam failure in a water containment system causing a small leak might not be critical, such a leak in a hazardous waste system is extremely critical, and consequently governmental regulatory agencies such as EPA require almost failsafe seam conditions.
Moreover, solvent and adhesive seams are subject to attack not only by the chemicals in the liner itself, but also to microbiological organisms which actually feed on the adhesive material. Hence, even if the seam is substantially perfect when installed, over a period of time it can lose its bonding strength and result in a seam failure.
There is a strong need and a strong desire by private contractors and by government agencies for the production of a thermal seam in the field comparable to a factory thermal seam, as it is well recognized that such a seam is far superior for all purposes to a solvent or adhesive seam.